1. Field of the Invention
This invention relates to a superconducting Josephson junction, circuit and method of manufacture and, more particularly, to an insulating barrier material having a chemical and structural composition generally compatible with that of the superconducting material.
2. Description of the Relevant Art
A superconductor-insulator-superconductor (SIS) Josephson junction is an electronic device consisting of two superconducting electrodes separated by a very thin insulating barrier. The thin insulating barrier permits the passage of supercurrents which tunnel through the barrier while maintaining zero voltage across the junction. Josephson devices switch very rapidly from zero voltage to a finite voltage level when current levels exceed a critical threshold value I.sub.c of supercurrents. The switching voltage is dependent on the superconductor material used, and is typically a few mV, as compared to the switching voltages of silicon and gallium arsenide transistors which are generally greater than 200 mV to 500 mV.
Because of their low operating voltages, Josephson devices can operate as low power, fast digital switches. Once current exceeds the threshold value I.sub.c (usually less than 1 mA), the device will switch rapidly from, e.g., an "off" state to an "on" state. Since the threshold current is quite low and the device exhibits low switching voltage, very little power is dissipated during operation. These properties, along with the ability to detect small amounts of magnetic fields or field gradients, make Josephson devices potentially useful in high performance computers, sensors and communication systems.
Many superconductors operate according to the principles of superconductivity analyzed by the well known Bardeen, Cooper and Schrieffer theory ("BCS theory") which predicted how the forces between electrons and surrounding atoms in the superconducting matrix material can lead to a pairing of electrons, called "Cooper pairs" resulting in zero resistance. An important part of the BCS theory is the calculation of the coherence length. Coherence length is a measure of the size of the Cooper pair and is defined as the fundamental size scale of superconductivity. The degree of superconductivity does not change over an arbitrarily short distance in the superconductor, but can change only over the characteristic distance of the coherence length.
To maintain tunneling of supercurrents through a Josephson junction, it is important that the properties of the superconductor be maintained to within a coherence length of the barrier. If regions of non-superconductivity in the superconducting electrode at the barrier interface exceed the coherence length, then the supercurrents are significantly decayed and may cease. Likewise the finite voltage characteristics of the tunnel junction are also dependent on the quality of the superconducting electrode at the barrier interface. The non-zero voltage behavior of the tunnel junction is a characteristic of the electronic states of the electrode material at the barrier interface. For superconductors, a gap in the electronic states exists creating a gap in the current-voltage (I-V) behavior of the junction with no current flow in the sub-gap region between -Vg and +Vg, as shown in FIG. 1. If, however, the electrode material at the barrier interface is not superconducting to within the coherence length, then non-superconducting characteristics are sampled in the I-V behavior of the device and this gap structure will be degraded or be eliminated completely, and sub-gap current may exist.
One important property of a high quality SIS Josephson device is its hysteretic behavior. Hysteretic behavior is largely dictated by the previous state of the device, whether it was in a zero voltage or a finite voltage state. As stated previously, FIG. 1 is an exemplary hysteretic current-voltage behavior of an ideal SIS Josephson tunnel junction at absolute zero temperature. As shown, if the junction is in the zero voltage state, it will remain there if the current does not exceed I.sub.c. As current is increased above I.sub.c, the supercurrent is quenched and the device experiences normal electron (not Cooper pair) flow through the barrier, which requires a voltage. Once normal flow is achieved, the corresponding voltage will remain, regardless of whether the current is above or below I.sub.c, until the current is brought all the way back to near zero. Thus, as current is increased from zero, the voltage across the Josephson device will appear only after current exceeds I.sub.c. However, once in the finite voltage state, voltage will remain even after the current drops to a much smaller current well below I.sub.c. An important property of hysteresis is the ability to latch or store a voltage even after the current drops below threshold. The modulating current must thereby drop to near zero before the logic state is lost. Several applications of superconducting tunnel junctions further require that the current flowing at voltages less than Vg ("energy gap") be small. According to the BCS theory, a sub-gap current flowing at V&lt;Vg would be zero for an ideal tunnel junction operated at absolute zero temperature. In some switching applications, the low sub-gap current is required to ensure that the device will switch to a relatively high voltage (near Vg) when switched out of the zero voltage state. When tunnel junctions are used for detecting or mixing millimeter waves, small sub-gap currents are required for high sensitivity.
Some compound superconductor materials display superconductive properties that are sensitive to small changes in the stoichiometry and the crystalline structure of the material. During manufacture of the Josephson junction, the superconductor electrode material may become depleted by diffusion of its components into the barrier. Depletion or out-diffusion of the superconductor components may cause a relative change in the material's stoichiometry for a distance exceeding the coherence length measured from the barrier which can thereby adversely affect the material's superconductive properties. Therefore, it becomes necessary to maintain the chemical compositions of such superconductors in order to preserve its superconductive properties to within a coherence length of the barrier. Likewise, the crystalline structure of some of these compound superconductors must also be maintained to within a coherence length of the barrier.
While there are many types of superconductive materials, there has been little success in making quality SIS Josephson tunnel junctions from some of the more desirable materials. Recent emphasis has been placed on finding materials with high critical temperature T.sub.c so that less expensive and more convenient means can be used to cool the superconductor. Examples of high T.sub.c materials are yttrium barrium copper oxide (YBa.sub.2 Cu.sub.3 O.sub.7-x), sometimes referred to as YBCO and having a T.sub.c equal to approximately 92K, and bismuth strontium calcium copper oxide (Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8), sometimes referred to as BSCCO and having a T.sub.c equal to approximately 105K. While the above material exhibit high T.sub.c, they do not produce a high quality SIS tunnel junction characteristics when incorporated into a Josephson device. Furthermore, the compound oxide superconductors described above must be in crystalline form for optimal performance. For the standard configurations of SIS tunnel junctions, it is desirable that the crystalline properties be preserved right up to the superconductor-barrier interface for both interfaces. This generally requires that the barrier be crystalline and have a good structural match with the lattice of the superconductor in the plane of the interface. Using a trilayer example, a barrier which is grown on the top surface of the bottom superconductor must have a base geometric size and shape which closely matches the bottom superconductor and, likewise, the top geometric size and shape of the barrier must closely match the top superconductor. If the barrier lattice does not match the superconductor lattice, then the resulting lattice strain and growth defects quench the superconducting properties in the interface region.
Recent studies by A. Lee, et al., "LaAlO.sub.3 -YBCO Multilayers, "IEEE Trans. on Mag., vol. 27, no. 2, March 1991; Q. Ying, et al., "YBa.sub.2 Cu.sub.3 O.sub.7-x -Y.sub.2 O.sub.3 System and in situ Deposition of Trilayer Heterostructures by Coevaporation,"Appl. Phys. Lett., 59 (23), December 1991; J. Barner, et al., "All a-axis Oriented YBa.sub.2 Cu.sub.3 O.sub.7-y -PrBa.sub.2 Cu.sub.3 O.sub.7-z -YBa.sub.2 Cu.sub.3 O.sub.7-y Josephson Device Operating at 80K," Appl. Phys. Lett., 59 (6), August 1991; and, I. Iguchi, et al., "Tunneling Characteristics of YBaCuO/MgO/Pb Planar Tunnel Junctions and Observation of Josephson Effect," Japanese Jr. of Appl. Phys., vol. 29, no. 4, April 1990 demonstrate that YBCO used with LaAlO.sub.3, Y.sub.2 O.sub.3, PrBa.sub.2 Cu.sub.3 O.sub.7 and MgO barriers, respectively, may produce a less than optimal Josephson device. As demonstrated, YBCO material, combined with an insulative barrier material may not produce a good SIS junction with desired hysteretic effect. It is commonly known that the coherence length of YBCO material is approximately in the range of two to five angstroms in a direction perpendicular to the Cu planes and 10 to 30 angstroms within the Cu planes. The relatively short coherence length for YBCO material is equally true for BSCCO materials. The lattice disruption of the superconductor-barrier interface which produces a region of non-superconducting YBCO (or BSCCO) that is larger than its coherence length (as measured from the barrier interface) may certainly be one explanation for the poor device performance of these materials. A material with a longer coherence length is therefore desirable in overcoming this obstacle.
The cubic perovskites Ba.sub.1-x K.sub.x BiO.sub.3 (or BKBO) and Ba.sub.1-x Rb.sub.x BiO.sub.3 (or BRBO) are superconductors with measured coherence lengths of approximately 50 angstroms. Coherence lengths of BKBO and BRBO are not only longer than those of YBCO or BSCCO but are also directionally isotropic. Longer coherence lengths gives BKBO and BRBO materials an advantages when used as electrodes in an SIS tunnel junction. The superconducting transition temperature T.sub.c of BKBO and BRBO is above 30K, lower than that of YBCO or BSCCO but twice as high as the T.sub.c (16K) of niobium nitride NbN, the highest T.sub.c currently known to produce high quality SIS Josephson tunnel junctions. With a transition temperature of 30K, a suitable operating range of the junction would be 12K-18K, which can be achieved with a closed-cycle refrigerator. Liquid helium would not be needed to chill the device, which is a significant advantage. Furthermore, there are many applications which require an operating temperature of 12K. It would not be an advantage in these cases to use superconducting materials with T.sub.c 's above 90K. In fact, it would be a disadvantage since, for higher T.sub.c materials, the switching voltages will be larger and therefore the power dissipated by high T.sub.c devices will also be larger.
An attempt to make all-BKBO SIS junctions was reported in a preprint by E. Hellman, et al., "Ba.sub.1-x K.sub.x BiO.sub.3 Sandwich-Type Tunnel Junctions Grown by Molecular Beam Epitaxy," submitted to Appl. Phys. Lett., Mar. 13, 1992. The barrier used for the Hellman et al. device was BaBi.sub.2 O.sub.y of undefined structure. Although BaBi.sub.2 O.sub.y may be a good lattice match to BKBO, BaBi.sub.2 O.sub.y may not be chemically compatible with BKBO. It is well known that alkali ions in solid solution are highly mobile and readily diffuse, especially at the elevated temperatures used to fabricate Josephson junctions and/or circuits. Typically, BKBO and BRBO are made in oxygen deficient atmospheres that lead to oxygen vacancies in materials during fabrication which further promotes ion diffusion. The diffusion from the concentrated BKBO region to the BaBi.sub.2 O.sub.y (which is initially void of K) may deplete the K ion concentration in the BKBO at the interface, thereby reducing or quenching its superconductive properties in this region.
The diffusion of mobile potassium ions from the BKBO top and bottom electrode and into the barrier which does not initially contain a potassium (K) compound is shown in FIG. 5. While an optimal alkali (or K) concentration, x, should remain near 0.4 throughout the BKBO structure, FIG. 5 illustrates possible non-superconducting regions, n, forming within the BKBO material near the barrier interface. As the concentration level decreases to a level less than that necessary to sustain superconductivity and for a distance n greater than the coherence length, regions of non-superconductivity arise which will destroy or pinch off the superconducting tunnel current. In addition, diffusion of impurity ions into the barrier may decrease its optimal insulating characteristics as well. A decrease in insulating characteristics may result in an SNS combination rather than a high quality SIS combination.
The problems associated with YBCO materials using conventional barrier combinations do not provide for high quality SIS Josephson devices having ideal or near ideal tunnel junction behavior. The relatively short coherence lengths of YBCO and BSCCO places very high demands on the quality of the barrier interfaces. Still further, the problems with potassium ion migration from the BKBO electrode to the barrier will adversely decrease the optimal concentrations of alkali ions within the BKBO superconductor or adversely inject alkali ions into non-alkali barriers.